While fingerprints are increasingly recorded at border crossings and are compared with databases, identification documents (personal identification documents) are additionally essential for international travel.
A conventional border crossing scenario, for example, is the digital recording of a document, usually a passport or identity card. This ID document is verified, and various security features must be applied, sometimes also with illumination wavelengths and detection wavelengths in the UV and IR wavelength range outside of the visible wavelength range. Further, RFID (radio frequency identification) data stored in the document are read out, a head shot is recorded, and at least one fingerprint with high requirements for quality is acquired.
The acquired biometric data are compared with the RFID data or with the passport photograph of the personal identification document. If all of the tests are passed, the user obtains a printout of the results or—in a different scenario—a door or barrier is opened.
In the procedure described above, important goals are speed, robustness and immunity to error of the system. A system which allows skin prints as well as documents to be recorded would appreciably increase the reliability of identification of persons based on the recorded skin print and the personal identification documents presented by the person and a matching of both results with data in a database. Further, a combined system could reduce the space requirement and, by inference, the range of activity of the control authorities and, accordingly, the processing time for travelers.
However, the recording of skin prints and documents is not only a typical procedure just at border crossings. In South American banks, for example, fingerprints must be taken before an account is opened. Subsequently, the identification card (ID card) is copied and filed. At the present time, there are also two different devices required for this.
There are similar scenarios with car rentals where, in addition to the fingerprint, the identity card or, instead of the identity card, the driver's license is scanned.
Likewise, when purchasing weapons in the United States, fingerprints are required for a background check, i.e., clearance by a law enforcement agency, in addition to a recording of the driver's license which is the de facto ID card in the United States.
Background checks, for which the combination of fingerprinting and document recording is necessary, are becoming increasingly important in the United States. Further, there is a growing demand for electronic visas for travelers to the European Union.
A typical field of application in which combined skin/document scanners are demanded is checking of access authorizations for personnel in security-related areas such as airports, nuclear power plants and oil refineries by scanning fingerprints and scanning the company identification card in which these fingerprints are stored.
A further scenario is mobile application such as mobile border checks, for example, in busses and trains. In this case, it is necessary to scan the identity card, to scan at least one fingerprint and to compare the same to that in the electronic chip of the identity card. Many other areas of mobile application are conceivable. In particular, the mobile solution would benefit from a combination of light, small technology for skin recording and document recording.
All of the requirement criteria necessitate a balanced and high-quality system design. In an optical system, this means that the requirements must be satisfied not only by the recording sensor, but also by the illumination and all other components necessary for imaging.
In systems for acquiring fingerprints and handprints, there are two systems of this type to be distinguished which are used for a verification, i.e., for a one-to-one comparison with stored fingerprints of a certain person, for example for entry control and access control, and other systems which are used for identification, i.e., for searching and recording comparison data in a database administered by government authorities, for example, for border controls, aviation security and for police use.
For the latter systems, there is a great number of demands with respect to quality, resolution and fidelity to the original of the recorded images of skin textures. Further, there is a high degree of standardization based on the requirements catalog of the FBI, on the one hand to ensure indisputable identification and, on the one hand, to allow comparison between datasets which were recorded by different systems. This requirements catalog includes six important parameters. First, the systems must have a resolution of either at least 500 ppi or at least 1000 ppi. There may be no certification below these resolutions. One of the most important parameters for certification is the contrast transfer function (CTF). In this respect, the requirements catalog defines exactly the minimum value that the CTF must have in the corresponding spatial frequencies in the image. As per FBI specifications, the contrast must achieve at least 25.8% for a 500 ppi system at 10 lp/mm, while for a 1000 ppi system at 20 lp/mm, a contrast of at least 28% must be achieved. Another important parameter is the signal-to-noise ratio (SNR) which must be at least 42 dB independent from the resolution. Further requirements include low distortion of <1%, the presence of at least 200 different grayscale values and a homogeneously illuminated image field both in the near pixel environment and in the image overall. The last parameter serves as a control for preventing image falsification. This means that unusual artefacts are explicitly looked for in the images to detect image manipulation.
Currently, the optical arrangements chiefly used for acquiring fingerprints and handprints with the high quality requirements corresponding to the requirements catalog of the FBI, for example for forensic purposes or for personal identification at border crossings, are based on the principle of frustrated total internal reflection (FTIR). In this case, owing to mechanical and optical requirements, a prism is used in which the provided surface for recording the print must be larger than the required surface for recording the print. The resulting size of the prism, commonly the largest component in the recording channel, decisively influences the minimum constructional size and minimum mass of a device of this kind. But on the other hand, the high image quality allows fast and reliable recognition and identification of persons, particularly for applications in which, along with forensic accuracy (FBI requirements), a high person throughput is expected, e.g., at border controls.
The disadvantages of these types of arrangements with prisms, apart from the size and mass, are the required complex mechanical components and a complicated assembly and adjustment. A recording of documents with the beam path used for fingerprint recording and based on FTIR is impossible. Therefore, the integration of this functionality in a prism-based device is only possible with additional recording channel as is described, e.g., in EP 2 120 182 B1. For the application scenarios described above, a second device is usually added for inputting the documents. However, this results in the problem that the user often confuses the placement surfaces of the two devices, i.e., the rate of erroneous operation is high. Moreover, both variants involve an enormous expenditure in terms of cost and space.
Miniaturized arrangements with imaging optics as disclosed, for example, in U.S. Pat. No. 7,379,570 B2 generally do not satisfy FBI requirements and still limit minimization of the fingerprinting devices because of the optical beam path. Recording documents with these systems is not conceivable because FTIR is also used in this case and a further beam path for image capture again conflicts with the approach of miniaturization.
Systems without conventional optical imaging promise a smaller and lighter type of construction. Ultrasonic sensors or piezo sensors as are known, e.g., from U.S. Pat. No. 4,394,773 A, and capacitive sensors as described, e.g., in U.S. Pat. No. 5,325,442 A allow imaging of documents but do not meet the quality standards of the FBI in the case of fingerprints. Membrane keyboards as known, for example, from US 2005/0229380 A1 do not meet FBI specifications and also do not allow recording of documents.
Approaches have already been described in US 2012/0321149 A1 for an optical method that makes do without conventional optically imaging elements. The arrangement disclosed therein, in which the finger is placed directly on a transparent plate above the substrate with the sensor array records a fingerprint. As in arrangements with prisms, the brightness profile corresponding to the fingerprint comes about in that the dermal ridges (also called papillary lines) resting on the surface of the transparent glass plate or plastic plate frustrate the internal reflection of the light from the light source, whereas there is no contact between skin and surface in the dermal valleys (also called papillary body lines) and the light from the light source is internally reflected there at the surface of the transparent plate. Accordingly, a negative image of the fingerprint is formed on the light-sensitive areas of the sensor array.
This solution requires that the upper transparent plate for placement of the finger has a minimum thickness so that the light can impinge on the light-sensitive elements of the sensor array when reflected at the underside of the transparent plate. Further, it is necessary that the illumination meets certain requirements with respect to incident direction and aperture angle or collimation, which appreciably increases the technical expenditure on illumination as well as the space requirement. The constructions for illumination presented cannot be implemented for large recording surfaces for more than one or two fingers or, if so, only at considerable expense.
A further concept for a flat construction without imaging optics is described in U.S. Pat. No. 7,366,331 B2. In this case, light is coupled laterally by means of areal illumination into the finger and, from the latter, also to the deposited part of the skin. The finger is in contact with a transparent layer between finger and sensor. Accordingly, light from the dermal ridges preferably couples into this layer and can be detected by the surface sensor in this way.
This concept requires illumination wavelengths within the transparency range of the finger (from 600 nm into the NIR range and IR range) and entails substantial problems with respect to ambient light. The proposed use of spectral filters with transparent IR windows and IR illumination mitigates these problems, but there is then a lower sensitivity of conventional sensors and higher absorption of the finger, which worsens the signal-to-noise ratio. When using narrowband spectral filters, the wavelength of the illumination must be adapted correspondingly, which incurs additional expenditure. The illumination concept is not suitable for recording documents. Further, the lateral illumination causes problems with the homogeneity of the lighting; in particular, it prevents the simultaneous capture of more than one finger because the fingers would shadow one another. Therefore, this concept is only suitable for recording one finger. Further, a light shield is needed in order to prevent portions of the illumination from directly reaching the sensor. The lateral illumination and light shield increase the size of the device and make it costlier, less flexible and more susceptible to errors.
A further concept for recording fingerprints without imaging is presented in WO 2015/005959 A1. This concept uses a special transparent layer which couples in the light reflected by objects on the side facing the object and couples it out again on the opposite side. The coupled-out light is in turn geometrically imaged in a conventional manner. In principle, the recording of documents seems possible with this concept. In this concept, the conventional imaging is generally carried out by a camera which requires a corresponding geometric beam path which substantially affects the minimum constructional size of the device. In this case, illumination can be applied by projection, but then has a further substantial space requirement. However, it can also be applied as additional layer below the above-mentioned special transparent layer. In this case, the imaging is carried out by the camera through this illumination layer, which imposes high requirements on this illumination layer as a result of the transparency needed for imaging. A fundamental problem with the concept is posed by interfering ambient light which is counteracted in this case only by software (subtraction of the image background). Accordingly, protection against overexposure is not guaranteed.